What is SOV, Share Of Voice in Online Advertising?

I am often asked about SOV in many Requests For Proposals (RFP) received by my media clients.  What is it and how is it calculated?  SOV stands for "Share Of Voice". In comparison to other advertisers, what is a certain clients' share of the total impressions you will deliver in a certain period of time online for them. If there were 2 advertisers the SOV would be 50/50.  If there were 4 advertisers, the share of voice for one client would be 25% if all advertisers were given the same the number of impressions in the same period of time.

It is very important that all publishers know the details of their online stats to be sure that they can answer questions like this when asked.  In addition, it is important that ad sales reps understand what SOV is and how it effects clients and their ad campaigns online.   

More online:  http://en.wikipedia.org/wiki/Share_of_voice